The theft took place at a bus stop on Ash Grove at 11 a.m. on Friday in the town of Blackburn, West Lothian.
A 43-year-old woman was standing at the bus stop looking at her phone when a man approached her from behind and threatened her with a knife before stealing the phone and making off through the nearby park.
The robber is described as white, around 5ft 7ins tall with a skinny build, red cheeks and a clean shaven face.
He was wearing a plain black hooded top with the hood pulled up, black jogging bottoms and black woollen gloves.
Detectives are keen to trace members of the public who spoke to the victim following the robbery and advised her where the police station was.
Detective Constable Alan Gibson said: “While the victim was uninjured, this incident left her extremely shaken and deeply upset.
Those with information can contact Police Scotland on 101 or alternatively, the charity Crimestoppers on 0800 555 111.